Air conditioner and control method thereof

ABSTRACT

An air conditioner including a plurality of compressors uses at least one oil separator for separating oil from an oil-mixed refrigerant discharged from the compressors. The air conditioner performs an oil balancing operation, during which some compressors operate and some do not. At least a portion of the oil separated by the oil separator is distributed to the compressors that are not operating during the oil balancing operation.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of Korean Patent Application No. 10-2007-0021118, filed on Mar. 2, 2007, which is hereby incorporated by reference as if fully set forth herein.

BACKGROUND

1. Field

The present application discloses an air conditioner including a plurality of compressors which is configured to supply balanced amounts of oil to the compressors, and a control method thereof.

2. Background

Generally, an air conditioner performs air conditioning using heat absorbed by or discharged from a refrigerant. The refrigerant is subjected to a refrigeration cycle of compression, condensation, expansion, and evaporation. One essential element of such an air conditioner is a compressor functioning to compress the refrigerant. The compressor is subjected to a high pressure, high temperatures, and the moving parts of the compressor may experience a high degree of friction. As a result, if the compressor is not properly lubricated, it can be easily damaged.

In order to ensure reliability of such a compressor, it is important to lubricate and cool the compressor. The lubrication and cooling is achieved by oil. The compressor should always contain a certain amount of oil.

However when the refrigerant is discharged from the compressor, after being compressed, a portion of the oil in the compressor may be mixed in with the refrigerant. As a result, the oil is discharged together with the refrigerant. As the compressor continues to operate, the amount of oil contained in the compressor decreases gradually. In conventional compressors, an oil separator is arranged in a discharge line connected to an outlet of the compressor. The separator separates the oil from the refrigerant and returns the separated oil to the compressor.

Where a plurality of compressors are used together in a single air conditioning system, typically connected in parallel, and where the amounts of refrigerant discharged from the respective compressors is different due to different capacities of the compressors, the amounts of oil contained in the respective compressors may not be balanced. As a result, the compressor or compressors containing an insufficient amount of oil may be damaged. In this case, there are considerable adverse affects on the performance of the overall system.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments will be described in detail with reference to the following drawings, in which like reference numerals refer to like elements, and wherein:

FIG. 1 is a diagram schematically illustrating a first embodiment of an air conditioner with multiple compressors;

FIG. 2 is an enlarged diagram of a portion of the air conditioning system shown in FIG. 1 which includes an accumulator, compressors and oil separators;

FIG. 3 is an enlarged diagram of an alternate embodiment of the same portion of the air conditioner as shown in FIG. 2;

FIG. 4 is an enlarged diagram of another alternate embodiment of the same portion of the air conditioner as shown in FIG. 2;

FIG. 5 is an enlarged diagram of another alternate embodiment of the same portion of the air conditioner as shown in FIG. 2; and

FIG. 6 is an enlarged diagram of still another alternate embodiment of the same portion of the air conditioner as shown in FIG. 2.

DETAILED DESCRIPTION

Reference will now be made in detail to the preferred embodiments, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.

The air conditioner shown in FIG. 1 includes an outdoor unit 1, a distribution unit 2, and an indoor unit 3. The air conditioner may perform only a cooling operation, only a heating operation, or the air conditioner may simultaneously perform both a cooling operation and a heating operation. The indoor unit 3 may comprise one indoor unit, or may comprise a plurality of indoor units. Accordingly, the distribution unit 2 may be dispensed with or may be required, in accordance with the configuration of the indoor unit(s) 3.

The outdoor unit 1 includes a compression unit 10, a heat exchanging unit 30, and an expansion device 40. Although not shown, the indoor unit 3 includes an indoor heat exchanger and an expansion valve. The distribution unit 2 can include a refrigerant line connecting the outdoor unit 1 and indoor unit 3, and various valves, to control a refrigerant flow between the outdoor unit 1 and the indoor unit 3.

Where the air conditioner is adapted to perform both a cooling operation and a heating operation (including both a simultaneous type or a switching type), a switching valve 20 is installed in the outdoor unit 1. The switching valve 20 switches the flow direction of a refrigerant discharged from the compression unit 10.

The air conditioner includes oil separation units 100 respectively connected to a plurality of compressors constituting the compression unit 10. Each oil separation unit 100 separates oil mixed with a refrigerant discharged from the associated compressor, and returns the separated oil to the associated compressor.

In FIG. 1, reference numeral “50” designates an accumulator. The accumulator temporarily stores the refrigerant to be returned to each compressor, to control the refrigerant amount in each compressor.

As shown in FIG. 2, the compression unit 10 includes three compressors 10 a, 10 b, 10 c. The number of oil separation units corresponds to the number of compressors. The oil separation units 101, 102, 103 may be connected to the compressors by a plurality of recovery lines, respectively. In order to balance the amounts of oil in the compressors, an oil balancer is provided. The oil balancer may include a bypass line communicating with the recovery lines and one or more bypass valves arranged in the bypass line.

In detail, the compression unit 10 includes a first compressor 10 a, a second compressor 10 b, and a third compressor 10 c. Of course, any number of compressors could be used, as long as the number of compressors is plural. Also, the plural compressors may comprise constant speed compressors or variable speed compressors. Alternatively, the plural compressors may comprise a combination of variable speed compressors and constant speed compressors.

Discharge lines 11 a, 11 b, and 11 c are connected to respective outlets of the compressors 10 a, 10 b, and 10 c. Compressed refrigerant flow through the discharge lines 11 a, 11 b, and 11 c after it is discharged from the compressors 10 a, 10 b, and 10 c.

The oil separation unit includes a first oil separator 101, a second oil separator 102, and a third oil separator 103. The first oil separator 101 is connected to the first discharge line 11 a, to communicate with the first compressor 10 a. The second oil separator 102 is connected to the second discharge line 11 b, to communicate with the second compressor 10 b. The third oil separator 103 is connected to the third discharge line 11 c, to communicate with the third compressor 10 c.

The supply lines, through which refrigerant flows from the accumulator 50 to the respective compressors includes a first supply line 12 a connected to the first compressor 10 a, a second supply line 12 b connected to the second compressor 10 b, and a third supply line 12 c connected to the third compressor 10 c.

The oil separators 101, 102, and 103 separate oil mixed with the refrigerant discharged from the compressors 10 a, 10 b, and 10 c. The separated oil is then returned to the compressors via the supply lines 12 a, 12 b, and 12 c, so as to return the separated oil to the compressors 10 a, 10 b, and 10 c, respectively. The air conditioner includes recovery lines 111, 112, and 113, through which oil flows from the oil separators 101, 102, and 103 to the compressors 10 a, 10 b, and 10 c. The recovery lines 111, 112, and 113 comprise a first recovery line 111 connected between the first oil separator 101 and the first supply line 12 a, a second recovery line 112 connected between the second oil separator 102 and the second supply line 12 b, and a third recovery line 113 connected between the third oil separator 103 and the third supply line 12 c.

Opening/closing valves 151, 152, and 153 are arranged in the recovery lines 111, 112, and 113, to open or close the recovery lines 111, 112, and 113, and thus, to control recovery of oil to the compressors 10 a, 10 b, and 10 c, respectively. The opening/closing valves 151, 152, and 153 comprise a first opening/closing valve 151 for controlling recovery of oil through the first recovery line 111, a second opening/closing valve 152 for controlling recovery of oil through the second recovery line 112, and a third opening/closing valve 153 for controlling recovery of oil through the third recovery line 113.

Preferably, expansion devices 141, 142, and 143 are arranged in the recovery lines 111, 112, and 113. The expansion devices act to expand (lower the pressure) oil which is being returned to the compressors 10 a, 10 b, and 10 c. The expansion devices 141, 142, and 143 comprise a first expansion device 141 for expanding oil recovered through the first recovery line 111, a second expansion device 142 for expanding oil recovered through the second recovery line 112, and a third expansion device 143 for expanding oil recovered through the third recovery line 113.

The reason why the expansion devices 141, 142, and 143 are installed is that there is a pressure difference between the refrigerant supplied from the accumulator 50 to each of the compressors 10 a, 10 b, and 10 c and the oil recovered from the associated oil separators 101, 102, or 103. The refrigerant from the accumulator 50 has a low pressure, whereas the oil separated from the compressed refrigerant has a high pressure.

The expansion devices reduce the pressure of oil flowing through the recovery lines 111, 112, and 113, and thus, reducing the pressure difference between each recovery line 111, 112, or 113 and the associated return line 12 a, 12 b, or 12 c. The pressure reduction function is also desirable because a reduction in temperature is also achieved during the expansion of the oil, and this helps to reduce an oil evaporation phenomenon to a certain degree.

The recovery lines 111, 112, and 113 are connected to a bypass line 120 such that the recovery lines 111, 112, and 113 communicate. In detail, the recovery lines 111, 112, and 113 comprise a first recovery line 111, a second recovery line 112, and a third recovery line 113 which all communicate with the bypass line 120. Accordingly, oil from the first oil separator 101 flowing through the first recovery line 111 may be sent through the bypass line 120 such that it is introduced into the second and third recovery lines 112 and 113, to thereby be recovered to the second and third compressors 10 b and 10 c. Thus, the oil recovered from any of the oil separators 101, 102, and 103 can be supplied, via the bypass line 120, to a compressor which contains an insufficient amount of oil.

Preferably, control valves 131 and 132 are arranged in the bypass line 120, to control oil flows among the recovery lines 111, 112, and 113. The control valves 131 and 132 comprise a first control valve 131 arranged in a portion of the bypass line 120 connecting the first and second recovery lines 111 and 112, and a second control valve 132 arranged in a portion of the bypass line 120 connecting the second and third recovery lines 112 and 113. The first control valve 131 controls an oil flow between the first recovery line 111 and the second recovery line 112. The second control valve 132 controls an oil flow between the second recovery line 112 and the third recovery line 113. For the control valves, solenoid valves may be used. Also, valves, which can adjust an opening degree thereof, may be used.

Now, a control method for the embodiment shown in FIG. 2 will be described. The control method for the first embodiment may include a normal operation step where oil separated by the plurality of oil separators 101, 102, and 103 is returned to their corresponding compressors 10 a, 10 b, and 10 c. The method would also include an oil balancing operation step for sequentially introducing the oil separated by the oil separators 101, 102, and 103 into the compressors 10 a, 10 b, and 10 c. A controller, which is included in the air conditioner may execute the normal operation step and the oil balancing operation step at intervals of a predetermined time. Alternatively, the controller may execute the normal operation step and the oil balancing operation step, based on data obtained in accordance with a measurement by a sensor for sensing the amount of oil of each compressor.

During normal operations, the controller closes the first and second control valves 131 and 132, and opens the first, second, and third opening/closing valves 151, 152, and 153, to enable the normal operation to be executed. In this state, oil separated by the first oil separator 101 is introduced into the first compressor 10 a. Oil separated by the second oil separator 102 is introduced into the second compressor 10 b, and oil separated by the third oil separator 103 is introduced into the third compressor 10 c. As the normal operation is repeatedly executed, the amount of oil contained in each compressor may vary.

Where a plurality of compressors having different capacities are used, an oil shortage may occur in one of the compressors. Even in the case in which the compressors have similar capacities, an oil unbalance phenomenon may occur because the driving times of the compressors may be different. When an unbalance occurs, an oil balancing operation is executed in the air conditioner. In detail, the controller of the air conditioner controls the oil balancer included in the air conditioner to supply balanced amounts of oil to the compressors.

The oil balancing operation step may include step A of operating at least one compressor while stopping the remaining compressors, and a step B of controlling the oil balancer to allow oil to be supplied to the non-operating compressors. The oil balancing operation step may further include a step C of operating at least one compressor including the compressor supplied with oil, and introducing oil into the compressors, except for the operating compressor. The oil balancing operation step may include repeating steps A, B, and C for all the plural compressors.

In the following description, it is assumed that during the normal operation step, all of the first, second, and third compressors were operated, and that an oil unbalance occurred among the compressors. Of course, the oil balancing operation can be executed for a compressor which did not operate during the normal operation step.

During the balancing operation, the controller of the air conditioner operates one of the plural compressors, while stopping the remaining compressors. The controller then controls the oil balancer such that oil separated by the oil separator associated with the operating compressor is supplied to one or both of the stopped compressors. In detail, the controller operates the first compressor 10 a, while stopping the second and third compressors 10 b and 10 c. The controller then opens the first control valve 131 and second opening/closing valve 152, while closing the second control valve 132, first opening/closing valve 151, and third opening/closing valve 153. Accordingly, oil separated by the first oil separator 101 is introduced into the stopped second compressor 10 b via the bypass line 120 and second recovery line 112. In this case, the controller operates the first compressor 10 a for a first predetermined time.

After the operation of the first compressor 10 a is completed, the controller maintains the first and third compressors 10 a and 10 c in a stopped state. Simultaneously, the controller operates the second compressor 10 b, which has been supplied with oil in the above procedure. Thereafter, the controller opens the second control valve 132 and third opening/closing valve 153, while closing the first control valve 131, first opening/closing valve 151, and second opening/closing valve 152. Accordingly, oil separated by the second oil separator 102 is introduced into the stopped third compressor 10 c via the bypass line 120 and third recovery line 113. In this case, the controller operates the second compressor 10 b for a second predetermined time.

The controller sequentially executes the above-described procedures for all compressors. Here, sequential operations of the compressors mean that all compressors operate sequentially in a given order. In detail, in the embodiment shown in FIG. 2, after the operation of the second compressor 10 b is completed, the controller maintains the second compressor 10 b and first compressor 10 a in a stopped state. Simultaneously, the controller operates the third compressor 10 c. Thereafter, the controller opens the first control valve 131, the second control valve 132 and first opening/closing valve 151, while closing the second opening/closing valve 152, and third opening/closing valve 153. Accordingly, oil separated by the third oil separator 103 is introduced into the first compressor 10 a via the bypass line 120 and first recovery line 111. In this case, the controller operates the third compressor 10 c for a third predetermined time.

The controller may execute the above-described procedures once for all compressors. If necessary, the controller may repeat the above-described procedures several times. After completion of the above-described procedures, the amounts of oil supplied to the compressors are balanced due to the following reasons.

Since different amounts of oil are contained in the compressors before the oil balancing operation, the ratio of oil mixed with a refrigerant in one compressor during operation of the compressor is different from those in the remaining compressors. For example, in a compressor containing a large amount of oil, a large amount of oil is mixed with a refrigerant. As a result, when that compressor is operated, a large amount of oil is discharged from the compressor, and this relatively large amount of oil is introduced into another one of the compressors. On the other hand, in a compressor containing a small amount of oil, a small amount of oil is mixed with a refrigerant. As a result, when that compressor operates, only a small amount of oil will be transferred over to another compressor. When all compressors operate sequentially, as explained above, the amounts of oil contained in the compressors tends to balanced out.

If sensors are installed in the respective compressors, to sense the amounts of oil contained in the respective compressors, the air conditioner according to the first embodiment can operate in the following manner. First, the controller determines which one of the compressors 10 a, 10 b, and 10 c, contains an insufficient amount of oil, based on sensing data from the sensors. If it is determined that the second compressor 10 b has an insufficient amount of oil, the controller then operates the first compressor 10 a, while stopping the second and third compressors 10 b and 10 c. Thereafter, the controller opens the first control valve 131 and second opening/closing valve 152, while closing the second control valve 132, first opening/closing valve 151, and third opening/closing valve 153. Accordingly, oil separated by the first oil separator 101 is introduced into the second compressor 10 b via the bypass line 120 and second recovery line 112. Thus, the second compressor 10 b is replenished with the oil.

If it is determined that the third compressor 10 c has an insufficient amount of oil, the controller will then execute a control operation to introduce oil separated by the first oil separator 101 into the third compressor 10 c, and thus, to replenish the third compressor 10 c with the oil. Similarly, if it is determined that the first compressor 10 a has an insufficient amount of oil, the controller will then execute a control operation to supply oil only to the first compressor 10 a.

Hereinafter, a procedure, in which an oil balancing operation is carried out in a second embodiment of the air conditioner, will be described with reference to FIG. 3.

In the air conditioner according to the second embodiment, the first and second control valves 131 and 132 are eliminated. That is, the first, second, and third recovery lines 111, 112, and 113 are always in communication by the bypass line 120. Of course, the control method according to this embodiment is also applicable to the case in which the first and second control valves 131 and 132 are used, as long as the first and second control valves 131 and 132 are maintained in an opened state. In the following description, it is assumed that, during the normal operation step, all the first, second, and third compressors were operated, and an oil unbalance occurred among the compressors.

For the oil balancing operation in the air conditioner according to the second embodiment, the controller first operates the first and second compressors 10 a and 10 b, while stopping the third compressor 10 c. The controller then closes the first and second opening/closing valves 151 and 152, while opening the third opening/closing valve 153. Accordingly, oil separated by each of the first and second oil separators 101 and 102 passes through the bypass line 120, and is then introduced into the third compressor 10 c via the third recovery line 113. In this case, the controller operates the first and second compressors 10 a and 10 b for a first predetermined time.

After the operations of the first and second compressors 10 a and 10 b are completed, the controller operates the second and third compressors 10 b and 10 c, while maintaining the first compressor 10 a in a stopped state. The controller closes the second and third opening/closing valves 152 and 153, while opening the first opening/closing valve 151. Accordingly, oil separated by each of the second and third oil separators 102 and 103 passes through the bypass line 120, and is then introduced into the first compressor 10 a via the first recovery line 111. In this case, the controller operates the second and third compressors 10 b and 10 c for a second predetermined time.

After the operations of the second and third compressors 10 b and 10 c are completed, the controller operates the third and first compressors 10 c and 10 a, while maintaining the second compressor 10 b in a stopped state. The controller closes the third and first opening/closing valves 153 and 151, while opening the second opening/closing valve 152. Accordingly, oil separated by each of the third and first oil separators 103 and 101 passes through to the bypass line 120, and is then introduced into the second compressor 10 b via the second recovery line 112. In this case, the controller operates the third and first compressors 10 c and 10 a for a third predetermined time.

The controller may execute the above-described procedures once. If necessary, the controller may repeat the above-described procedures several times. After completion of the above-described oil balancing operation, the controller opens the first, second, and third opening/closing valves 151, 152, and 153, to enable a normal operation to be executed. In the normal operation, oil emerging from the first oil separator 101, oil emerging from the second oil separator 102, and oil emerging from the third oil separator 103 will be mixed in the bypass line 120.

Of course, where the control valves 131 and 132 are used, they are opened in the oil balancing operation, but are closed in the normal operation.

If sensors are installed in the respective compressors, to sense the amounts of oil contained in the respective compressors, the controller will not operate a compressor which contains an insufficient amount of oil. Instead, the controller will operate at least one of the remaining compressors and control the opening/closing valves to supply oil to the compressor containing an insufficient amount of oil.

For example, when the first compressor 10 a contains an insufficient amount of oil, the controller does not operate the first compressor 10 a, but operates the second compressor 10 b, which contains a sufficient amount of oil. Thereafter, the controller closes the second and third opening/closing valves 152 and 153, while opening the first opening/closing valve 151.

Accordingly, oil separated by the oil separator 102 associated with the operating second compressor 10 b is introduced into the first compressor 10 a via the first recovery line 111. In this case, the sensor of the first compressor 10 a senses the amount of oil in the first compressor 10 a, to determine the operating time of the second compressor 10 b.

Hereinafter, an oil balancing operation carried out in a third embodiment will be described with reference to FIG. 4. The air conditioner according to this embodiment eliminates the opening/closing valves, but additionally includes a second bypass line 121 for connecting the third recovery line 113 and first recovery line 111. A third control valve 133 is arranged in the second bypass line 121, to control a flow of oil passing through the second bypass line 121. In the following description, an oil balancing operation is executed when it is assumed that, during the normal operation step, all of the first, second, and third compressors were operated, and an oil unbalance occurred among the compressors.

The controller first operates the first compressor 10 a, while stopping the second and third compressors 10 b and 10 c. The controller then closes the second and third control valves 132 and 133, while opening the first control valve 131. Accordingly, oil separated by the first oil separator 101 is partially introduced into the first compressor 10 a via the first recovery line 111. The remaining portion of the separated oil is introduced into the second compressor 10 b via the bypass line 120. In this case, the controller operates the first compressor 10 a for a first predetermined time.

After the operation of the first compressor 10 a is completed, the controller operates the second compressor 10 b, while maintaining the third and first compressors 10 c and 10 a in a stopped state. The controller closes the third and first control valves 133 and 131, while opening the second control valve 132. Accordingly, oil separated by the second oil separator 102 is partially introduced into the second compressor 10 b via the second recovery line 112. The remaining portion of the separated oil is introduced into the third compressor 10 c via the bypass line 120. In this case, the controller operates the second compressor 10 b for a second predetermined time.

After the operation of the second compressor 10 b is completed, the controller operates the third compressor 10 c, while maintaining the first and second compressors 10 a and 10 c in a stopped state. The controller closes the first and second control valves 131 and 132, while opening the third control valve 133. Accordingly, oil separated by the third oil separator 103 is partially introduced into the third compressor 10 c via the third recovery line 113. The remaining portion of the separated oil is introduced into the first compressor 10 a via the second bypass line 121. In this case, the controller operates the third compressor 10 c for a third predetermined time.

The controller may execute the above-described procedures once. If necessary, the controller may repeat the above-described procedures several times. After completion of the above-described oil balancing operation, the controller executes a control operation for a normal operation of the air conditioner. That is, the controller closes the first, second, and third control valves 131, 132, and 133, and operates at least one of the first, second, and third compressors 10 a, 10 b, and 10 c, which is required for the normal operation. During normal operations, oil emerging from the first oil separator 101 is introduced into the first compressor 10 a. Oil emerging from the second oil separator 102 is introduced into the second compressor 10 b, and oil emerging from the third oil separator 103 is introduced into the third compressor 10 c.

If sensors are installed in the respective compressors, to sense the amounts of oil contained in the respective compressors, the controller will not operate the compressor which contains an insufficient amount of oil, but will operate at least one of the remaining compressors and control the control valves to supply oil to the compressor containing an insufficient amount of oil.

For example, when the first compressor 10 a contains an insufficient amount of oil, the controller does not operate the first compressor 10 a, but operates the second compressor 10 b, which contains a sufficient amount of oil. The controller closes the second and third control valves 132 and 133, while opening the first control valve 131.

Accordingly, oil separated by the second oil separator 102 is partially introduced into the first compressor 10 a via the bypass line 120 and first recovery line 111. The remaining portion of the separated oil is introduced into the second compressor 10 b. In this case, the sensor of the first compressor 10 a senses the amount of oil in the first compressor 10 a during the operation of the second compressor 10 b, to determine the operating time of the second compressor 10 b.

Hereinafter, a fourth embodiment of the air conditioner will be described with reference to FIG. 5. The air conditioner according to the fourth embodiment of the present invention includes two compressors. A control procedure for the air conditioner according to the fourth embodiment will now be described in detail. In the following description, an oil balancing operation is executed when it is assumed that, during the normal operation step, both the first and second compressors were operated, and an oil unbalance occurred between the compressors.

The controller first operates one of the two compressors. For example, as shown in FIG. 5, the controller operates the first compressor 10 a, while stopping the second compressor 10 b. Thereafter, the controller closes the first opening/closing valve 151, while opening the second opening/closing valve 152 and control valve 131. As the first compressor 10 a operates, oil separated by the first oil separator 101 is introduced into the second compressor 10 b via the bypass line 120 and second recovery line 112. In this case, the controller operates the first compressor 10 a for a first predetermined time.

After the operation of the first compressor 10 a is completed, the controller maintains the first compressor 10 a in a stopped state, while operating the second compressor 10 b. Thereafter, the controller closes the second opening/closing valve 152, while opening the first opening/closing valve 151 and control valve 131. Accordingly, oil separated by the second oil separator 102 is introduced into the first compressor 10 a via the bypass line 120 and first recovery line 111. In this case, the controller operates the second compressor 10 a for a second predetermined time.

When the oil balancing operation is completed, the controller closes the control valve 131, while opening the first and second opening/closing valves 151 and 152, to enable a normal operation to be executed.

If sensors are installed in respective compressors, to sense the amounts of oil contained in respective compressors, the air conditioner according to the fourth embodiment can operate in the following manner. First, if it is determined that the first compressor 10 a contains an insufficient amount of oil, based on sensing data from the sensors, the controller stops the first compressor 10 a, and operates the second compressor 10 b. The controller opens the first opening/closing valve 151, closes the second opening/closing valve 152, and opens the control valve 131. As the second compressor 10 b operates, oil separated by the second oil separator 102 is introduced into the first compressor 10 a via the bypass line 120 and first recovery line 111. Thus, the second compressor 10 b is replenished with oil.

On the other hand, if it is determined that the second compressor 10 b contains an insufficient amount of oil, the controller stops the second compressor 10 b, and operates the first compressor 10 a. The controller opens the second opening/closing valve 152 and control valve 131, and closes the first opening/closing valve 151. As the first compressor 10 a operates, oil separated by the first oil separator 101 is introduced into the second compressor 10 b via the bypass line 120 and second recovery line 112. Thus, the second compressor 10 b is replenished with oil.

When the oil balancing operation is completed, the controller closes the control valve 131, while opening the first and second opening/closing valves 151 and 152, to enable a normal operation to be executed.

Hereinafter, a fifth embodiment of the air conditioner will be described with reference to FIG. 6. The air conditioner according to this embodiment includes a single oil separator 200. An oil balancing operation in the air conditioner according to the fifth embodiment will now be described. In the following description, an oil balancing operation is executed when it is assumed that during the normal operation step, all the first, second, and third compressors were operated, and an oil unbalance occurred among the compressors.

The controller first operates the first compressor 10 a, while stopping the second and third compressors 10 b and 10 c. The controller then closes the first and third opening/closing valves 151 and 153, while opening the second opening/closing valve 152. Accordingly, oil separated by the oil separator 200 is introduced into the second compressor 10 b via the second recovery line 112. In this case, the controller operates the first compressor 10 a for a first predetermined time.

After the operation of the first compressor 10 a is completed, the controller operates the second compressor 10 b, while maintaining the third and first compressors 10 c and 11 a in a stopped state. Thereafter, the controller closes the second and first opening/closing valves 152 and 151, while opening the third opening/closing valve 153. Accordingly, oil separated by the oil separator 200 is introduced into the third compressor 10 c via the third recovery line 113. In this case, the controller operates the second compressor 10 b for a second predetermined time.

After the operation of the second compressor 10 b is completed, the controller operates the third compressor 10 c, while maintaining the first and second compressors 10 a and 10 b in a stopped state. Thereafter, the controller closes the third and second opening/closing valves 153 and 152, while opening the first opening/closing valve 151. Accordingly, oil separated by the oil separator 200 is introduced into the first compressor 10 a via the first recovery line 111. In this case, the controller operates the third compressor 10 c for a third predetermined time.

The controller may execute the above-described procedures once. If necessary, the controller may repeat the above-described procedures several times. After completion of the above-described oil balancing operation, the controller opens the first, second, and third opening/closing valves 151, 152, and 153, to enable a normal operation to be executed. In the normal operation, oil emerging from the oil separator 200 is introduced into the compressors via the first, second and third recovery lines, respectively.

Of course, if there are sensors for sensing amounts of oil contained in respective compressors, the controller will not operate the compressor which contains an insufficient amount of oil, but will operate at least one of the remaining compressors and control the opening/closing valves to supply oil to the compressor containing an insufficient amount of oil.

For example, when the first compressor 10 a contains an insufficient amount of oil, the controller does not operate the first compressor 10 a, but operates the second compressor 10 b, which contains a sufficient amount of oil. The controller closes the second and third opening/closing valves 152 and 153, while opening the first opening/closing valve 151. Accordingly, oil separated by the oil separator 200 is introduced into the first compressor 10 a via the first recovery line 111. In this case, the sensor of the first compressor 10 a senses the amount of oil in the first compressor 10 a, to determine the operating time of the second compressor 10 b.

As apparent from the above description, the air conditioners and control methods thereof can effectively perform oil balancing for a plurality of compressors, to achieve an enhancement in the operation reliability of the compressors. In particular, during the oil balancing operations, all the compressors, which were operated in a normal operation, are sequentially operated. Thus, oil balancing can be effectively achieved.

In the oil balancing operations, at least a portion of the oil separated from an oil-refrigerant mixture discharged from the compressor which operates during the oil balancing operation is supplied to at least one of the stopped compressors. Accordingly, oil balancing can be effectively achieved.

An air conditioning system as described above can include both high-pressure type and low-pressure type compressors, connected in series, and are independently controlled. Accordingly, there is an advantage in that it is possible to easily adjust the compression ratio of the compression system. Also, the high-pressure type and low-pressure type compressors are connected using an oil connecting line. Accordingly, it is possible to equally distribute oil to the high-pressure type and low-pressure type compressors, through a simple method using the hydrostatic pressure of oil.

Any reference in this specification to “one embodiment,” “an embodiment,” “example embodiment,” etc., means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of such phrases in various places in the specification are not necessarily all referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with any embodiment, it is submitted that it is within the purview of one skilled in the art to effect such feature, structure, or characteristic in connection with other ones of the embodiments.

Although a number of illustrative embodiments have been described, it should be understood that numerous other modifications and embodiments can be devised by those skilled in the art that will fall within the spirit and scope of the principles of this disclosure. More particularly, variations and modifications are possible in the component parts and/or arrangements of the subject combinations which would fall within the scope of the disclosure, the drawings and the appended claims. In addition to variations and modifications in the component parts and/or arrangements, alternative uses will also be apparent to those skilled in the art. 

1. An air conditioner, comprising: a plurality of compressors; at least one oil separator for separating oil from a mixture of oil and refrigerant discharged from the compressors; and an oil balancer that operates, during an oil balancing operation, to send oil from the at least one oil separator to a compressor that is not operating during the oil balancing operation.
 2. The air conditioner of claim 1, further comprising a plurality of recovery lines, wherein each recovery line connects the at least one oil separator to an intake of one of the compressors.
 3. The air conditioner of claim 2, further comprising a plurality of recovery valves, wherein each recovery valve is arranged on one of the recovery lines to control a flow of oil passing through the recovery line to the intake of the associated compressor.
 4. The air conditioner of claim 3, further comprising a controller that controls the recovery valves.
 5. The air conditioner of claim 4, wherein during the oil recovery operation, one of the compressors operates and one of the compressors does not operate, and wherein the controller controls the plurality of recovery valves so that at least a portion of the oil separated by the at least one oil separator is directed to the non-operating compressor.
 6. The air conditioner of claim 2, wherein the at least one oil separator comprises a plurality of oil separators, wherein one oil separator is provided for each compressor, and wherein each recovery line couples one of the oil separators to its corresponding compressor.
 7. The air conditioner of claim 6, wherein the oil balancer includes a bypass line that allows oil to flow between the plurality of recovery lines.
 8. The air conditioner of claim 7, wherein the oil balancer further includes at least one bypass valve arranged in the bypass line, to control a flow of oil through the bypass line.
 9. The air conditioner of claim 8, further comprising a controller that controls the at least one bypass valve, wherein during the oil recovery operation, one of the compressors operates and one of the compressors does not operate, and wherein the controller controls the at least one bypass valve so that at least a portion of the oil separated by the oil separator of the operating compressor is directed to the non-operating compressor.
 10. The air conditioner of claim 7, further comprising a plurality of recovery valves, wherein each recovery valve is arranged on one of the recovery lines to control a flow of oil passing through the recovery line to the intake of the associated compressor.
 11. The air conditioner of claim 10, further comprising a controller that controls the plurality of recovery valves, wherein during the oil recovery operation, one of the compressors operates and one of the compressors does not operate, and wherein the controller controls the plurality of recovery valves so that at least a portion of the oil separated by the oil separator of the operating compressor is directed to the non-operating compressor.
 12. The air conditioner of claim 11, wherein the oil balancer further includes at least one bypass valve arranged in the bypass line, to control a flow of oil through the bypass line, and wherein the controller also controls the at least one bypass valve.
 13. The air conditioner of claim 2, further comprising a plurality of expansion devices, wherein each expansion device operates to reduce a pressure of oil flowing through one of the recovery lines.
 14. The air conditioner of claim 1, wherein the compressors operate sequentially during the oil balancing operation.
 15. The air conditioner of claim 1, further comprising a plurality of sensors, wherein each sensor detects an amount of oil contained in one of the compressors.
 16. The air conditioner of claim 15, further comprising a controller that controls the oil balancer to supply oil to a compressor which contains an insufficient amount of oil, based on sensing data from the sensors.
 17. An air conditioner comprising: a plurality of compressors; at least one oil separator for separating oil from a mixture of oil and refrigerant discharged from the compressors; and an oil balancer that operates, during an oil balancing operation, to distribute balanced amounts of oil to the compressors, wherein the compressors operate sequentially during the oil balancing operation.
 18. The air conditioner of claim 17, wherein during the oil balancing operation, at least one of the compressors operates, and at least one of the compressors does not operate, and wherein the oil balancer supplies oil to all of the compressors except for the compressor that is operating.
 19. The air conditioner of claim 17, wherein during the oil balancing operation, at least one of the compressors operates, and at least one of the compressors does not operate, and wherein the oil balancer supplies oil to one of the non-operating compressors.
 20. The air conditioner of claim 17, wherein the at least one oil separator comprises a plurality of oil separators, wherein each oil separator corresponds to one of the compressors, and wherein during normal operations, the oil separated in each of the oil separators is returned to an inlet of its corresponding compressor.
 21. A method for controlling an air conditioner that includes a plurality of compressors and at least one oil separator that separates oil from a mixture of oil and refrigerant discharged from the compressors, comprising: conducting a normal operation step, during which, oil separated by the at least one oil separator is returned to the operating ones of the plurality of compressors; and conducting an oil balancing operation, during which, the oil separated by the at least one oil separator is returned to less than all of the compressors.
 22. The method of claim 21, wherein during the oil balancing operation, oil is not returned to one of the compressors that is operating during the oil balancing operation.
 23. The method of claim 21, wherein during the oil balancing operation, one of the compressors operates and one of the compressors does not operate, and wherein oil is returned to the compressor that is not operating.
 24. The method of claim 23, wherein during the oil balancing operation, oil is returned to both the non-operating compressor, and at least one of the operating compressors.
 25. The method of claim 21, wherein during the oil balancing operation, the compressors are operated sequentially, and wherein during each sequential operation, oil is not returned to the compressor that is operating.
 26. The method of claim 21, wherein during the oil balancing operation, the compressors are operated sequentially, and wherein during each sequential operation, only a portion of the oil being discharged by the operating compressor is returned to the operating compressor.
 27. The method of claim 21, wherein the air conditioner comprises a plurality of oil separators, wherein each oil separator corresponds to one of the compressors, wherein recovery lines connect each oil separator to its corresponding compressor, wherein a bypass line allows oil to flow between the recovery lines, and wherein during the oil balancing operation, oil from the oil separator of an operating compressor is sent through the bypass line to at least one of the non-operating compressors.
 28. A method for controlling an air conditioner that includes a plurality of compressors and at least one oil separator that separates oil from a mixture of oil and refrigerant discharged from the compressors, comprising: conducting a normal operation step, during which, oil separated by the at least one oil separator is returned to each of the plurality of compressors that are operating; and conducting an oil balancing operation, during which, at least one of the compressors is not operating, wherein the oil separated by the at least one oil separator is returned to the non-operating compressor. 